Damped Track on object in armature moves in unexpected and irregular way
A bit weird answering my own question but I found a solution. : )
I now believe the problem was due to the piston and cylinder being each-others Damped Track target. I suspect micro translations of their origins when moving caused it, but that is a guess.
I added an object, child of the platform, at the location of the cylinder origin. And also an object, child of the arm, at the location of the piston origin. I used those objects as targets for the Damped Track constrains, instead of the piston and cylinder objects themselves. It seems this has negated the glitching or jittering effect entirely.

Even though i found a satisfactory fix myself, still, thanks to anyone who gave this problem some thought.  : )